Croteam was building it for a marketing campaign (not unlike the working Nosulus Rift gimmick). And it built more than one. Croteam brought at least two working Vive minigun props to the Vive booth at EGX. A Vive controller stuck in the side barrel the minigun is used to track the prop, but it does feature a few buttons and a trigger on the handle. Croteam even went so far as to motorize the barrel array so that it spins when you fire the gun. 
 
http://www.tomshardware.com/news/serious-sam-mixed-reality-minigun,32749.html 
 
hmmmm.... working miniguns to play with 
get the real Feel of FPS
 

Sorta silly. The prop controller interests me zero. The mixed reality greenscreen room and the tech used to video it in real time on the other hand interests me. Room scale VR with mixed reality live stream of games that I actually want to play would be fun. We never get to see how mixed reality setups are enabled though. It's like they use tech voodoo or something that us regular end-users could only dream of doing in their Twitch TV streams.